Aqueous systems are increasingly being used in the coating of textiles and leather. Requirements such coating systems have to meet are in particular high resistances to chemicals and water, high mechanical stability and high tensile strength and ultimate tensile strength and adhesion. These requirements are substantially met by polyurethane dispersions. The various possibilities for the preparation of such dispersions was summarized, for example, by D. Dieterich in a review article [D. Dieterich, Prog. Org Coatings 9, 281 (1981)].
As [lacuna] in U.S. Pat. No. 5,041,494, U.S. Pat. No. 5,354,808 and U.S. Pat. No. 5,554,686 and in EP-A 0 924231 and EP-A 0 924230, the chemical and mechanical stability of coatings of polyurethane-polyurea dispersions, referred to below as PU dispersions, can be improved if the PU dispersions are modified with alkoxysilane groups.
A further improvement of the resistance to chemicals and hardness of alkoxysilane-modified PU dispersions is achieved in combination with silica sols and is described in detail in U.S. Pat. No. 5,945,476, U.S. Pat. No. 5,952,445, U.S. Pat. No. 5,859,118, U.S. Pat. No. 5,932,652 and EP-A 0 924 232.
However, those combinations of alkoxysilane-modified PU dispersions and silica sols which are known to date from the prior art have the disadvantage that the preparation of the alkoxysilane-modified PU dispersions is very much more complicated compared with conventional polyurethane dispersions. Furthermore, the silanes required for the preparation of the alkoxysilane-modified PU dispersions are comparatively difficult to obtain and are very expensive, with the result that commercial use is restricted to special areas.
It was therefore the object of the present invention to provide alternative PU dispersions for the area of textile and leather coating which, on the one hand, have the advantages of the silane-modified products, namely the improved resistances to chemicals in comparison with conventional PU dispersions in combination with good swelling behavior in water and improved mechanical properties and in the preparation of which, on the other hand, it is possible to dispense with expensive process steps involving starting compounds, such as alkoxysilanes, which are difficult to obtain.
Surprisingly, it was found that PU dispersions which do not have alkoxysilane groups, in combination with silica sols, also correspond to the property profile described above. In addition, it was found that these dispersions have, inter alia, substantially improved water vapor impermeability in comparison with conventional PU dispersions and their gloss levels do not change. Moreover, the dispersions according to the invention are more readily obtainable than the water vapor permeable PU coating materials disclosed in the prior art (DE 4 236 569A1) and have better water vapor permeability in combination with equally good swelling behavior.